The present invention concerns a portion of a video display system. The video display system may be of the type illustrated in the above-referenced inventions.
In such a video display system it is desired to have the ability to superimpose video graphics upon an external video source, such as a transmitted TV signal, cable TV signal, or video recorder signal. Such a video display system has utility, for example, in home computers, videotext systems, close-captioned TV systems, and arcade games. At present, such capability is known only for a non-interlaced external video signal, which in effect excludes normal NTSC or PAL external TV transmissions, since they are of the interlaced type.
In a non-interlaced system, the display information is simply repeated on corresponding lines of the two vertical sweeps or fields representing each frame. That is, line 0 of the even field repeats line 1 of the odd field. There is no difference in information between the even and odd fields. Thus in a video display system in which graphics are superimposed upon a non-interlaced external video signal, it is not necessary to know which field is currently being displayed. If the system is operating in the NTSC transmission system (but not receiving interlaced lines), this provides frame resolution of only 200 lines, or 250 lines in the PAL transmission system.
In order to provide greatly improved frame detail in a video display system in which graphics information is superimposed upon an external TV signal, it is desirable that the system be capable of working with the standard NTSC or PAL interlaced signals, in which the video information displayed during the even field is different than that displayed during the odd field. This in effect doubles the resolution of the information which can be displayed. In such a system it is necessary to know which field of the external video frame signal is currently being displayed.